The dating of the ancient Indus civilization was actually done through a Letter to the Editor of the Illustrated London News by Professor A.H. Sayce on September 27, 1924. This was the issue right after Marshall's sensational article on the discovery of the mysterious new cities of Harappa and Mohenjo-daro seven days earlier. Sayce recognized the seals as identical to those he had found in Sumer at layers from around 2300 BCE. With one letter to the editor, India's history had just been pushed back 2,000 years in Western eyes.
Nayanjot Lahiri's book Finding Forgotten Cities: How the Indus Civilization Was Discovered tells the incredible story of these early pioneers.